Yes, showing a GUI is done via UnityGUI - if you had looked in the manual, you would have found it.

Have a look at the GUI basics: http://unity3d.com/support/documentation/Components/gui-Basics.html. What you probably look for is

GUI.Label(Rect(10,10,200,20), myvar);

.ToString() is also available in JavaScript. That will convert your boolean to a string saying either "true" or "false".

you can show the content of a variable in GUI using a Label. you should use the ToString method in C# to get the representing string of your variable. if you want to show te value of some properties of your own component just override the ToString method and return the string that you want.